That's not quite what I was looking for: I'd like to present the user with a preview of what the merge result (handwriting + book page) would look like. This involves generating a single PDF page using existing routines for merging and displaying it in the plugin interface. There doesn't have to be any user interaction with the page on display - a PDF rendered on a QWidget would be more that sufficient.

The goal is to provide the user with the possibility to change the x and y offsets and scaling of handwriting.